Certainly there is no prescribed order, but we assume that a given
implementation always uses the same order. Otherwise most of our test
comparisons in the test suite are broken, and features like
PagedResults cannot be implemented.

Back-meta, which is the reason that "async" appeared so early in
slapo-glue(5), is a clear violation of the above... For that reason,
in most of the back-meta tests, I need to use -S "" to ensure the same
ordering in the results; as for the sizelimit, that's a completely
different issue. I think you're absolutely right for a slapo-glue
implementation that deals with data that is internally stored. In
that case, the possibility to guarantee that the order is preserved in
the implementation may be of value (can we change that order when
changing versions? I think it already occurred at least once for
backglue). But when dealing with data stored outside (as happens with
the proxy backends) I wouldn't count much on the ordering. Even when
plain back-ldap is used to proxy non-OpenLDAP software I think there's
no way we can guarantee for the ordering of data returned from other
vendor's implementations.

Right.

For now, I've dropped any mention of async from the subordinate
description in slapd.conf(5).